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Full text of "Practical Organic Chemistry"

120

PRACTICAL  ORGANIC  CHEMISTRY

Eotation of Ethyl Tartrate.—Fill a 200 mm. pohrimeter-
tube with the tartrate prepared. Whilst it is settling determine
the zero of the instrument, and if it does not coincide with tlie
zero of the graduated circle, a corresponding correction must l>e
introduced in the subsequent observations. The tube is then
placed in the instrument, and the angle of rotation determined
by turning the analyser N until equality of illumination is esUtl>~
lished in the two halves of the field. In making polarimet ric:
observations reliance should not be placed on a single setting °f
the instrument, but at least five or six readings should be ma.de,
which, with a good instrument, should not differ by more tli.m
four or five minutes. The temperature at the time of observa-
tion must be noted, and the density determined cither at lliiit
temperature or at two or three, other temperatures, and tlie
required density found by extrapolation,

Example :—

Temp.
		•    Length.
			a
	,/ i -2059
	[a],,
 7*66

2O°
	
	
	199*85 mm.
		18° 28'
		
[«]£ = 7'66° [a]}? - 7-47° [a]g = 7-27°
				- Anschiitz, Pictet, /Av., 1880, 13, 1177.
 }
			

	L«lb4
 Mb8
 MS
		= 7-07 = 6-86° = 6-66°
	/•   By extrapolation.
			
Eotation of Tartaric Acid.—The specific rotation of a
dissolved substance can be calculated from the rotation of tlit;
solution if the concentration is known. The formula to be u.*>tMi
for this purpose is :—

where a is the angle of rotation of solution, / the length of 11 it!
tube, and c the concentration, i.e., the weight in grams of tlie
dissolved substance contained in 100 c.c. of solution. "I'll ft

dissolved  substance  contained

formula [a]D = A^£ may also 1
Ip a

where p is the percentage (by weight) of substance in solution,

formula [a]o = •.-—- may also be used (it is, in fact, identic.11 j,
Ip a